1. Field of the Invention
The present invention relates to an optical receiving device, a free space optics transmission apparatus and a receiving apparatus used in free space optics transmission. For example, the present invention relates to an optical receiving device, a free space optics transmission apparatus and a receiving apparatus that use a photonic crystal to collect light.
2. Related Art of the Invention
In free space optics transmission, it is ideal to align the optical axis of a transmitter with that of a receiver. However, there is practically a static axis drift resulting from misalignment of a fixed transmitter with a fixed receiver or a dynamic axis drift when at least one of the transmitter and the receiver is a mobile apparatus.
Axis drift causes various problems, such as S/N degradation due to reduction in optical receiving level, reduction in transfer rate due to increase in error rate and increase in transmission power.
Furthermore, the fact that the faster an optical receiving element responds, the smaller the optical receiving area leads to greater impact of the axis drift. The axis drift described herein includes both positional and angular drifts.
In free space optics transmission, since the beam diameter increases to some extent during propagation in a free space, the angular drift is particularly more problematic than the positional drift. However, a conventional lens or the like for focusing light to a small-area optical receiving element has a dispersion plane that is an uncontrollable curved plane, so that the focusing position is sensitive to variation of the angle of incidence, resulting in degradation in gain.
In recent years, the study on photonic band engineering, in which a crystal lattice of a photonic crystal is designed to freely control a dispersion plane, has been active and some of the study results have been applied to imaging optical systems.
For example, there is an imaging optical system including a light collecting portion that collects incident light, a photonic crystal having a flat dispersion plane that propagates the incident light within a predetermined wavelength range in a fixed direction independent of the state of incidence (angle and position) and an optical receiving element (see Japanese Patent Laid-Open No. 2005-203676, for example).
FIGS. 10A and 10B are cross-sectional views showing a unit pixel of a solid-state imaging element disclosed as an imaging optical system in Japanese Patent Laid-Open No. 2005-203676. FIG. 10A shows the behavior of vertical incident light, and FIG. 10B shows the behavior of obliquely incident light.
The incident light comes from above a solid-state imaging element 200. An optical receiving element 102 (silicon p-i-n structure) is formed on a silicon substrate 101, and an aluminum light-blocking layer 103 for preventing smear is provided in the area except the opening above the optical receiving element 102. A photonic crystal 107 is formed such that the light-blocking layer 103 is embedded in the photonic crystal 107, and an acryl layer 104, a color filter layer 105 and a microlens 106 are formed above the photonic crystal 107. The focal length of the microlens 106 is adjusted such that the focal point coincides with the optical receiving surface of the optical receiving element 102.
FIG. 11 shows a specific structure of the photonic crystal 107 for blue light. In FIG. 11, light is incident at various angles from the left and exits from the right side (the optical receiving surface side). The photonic crystal 107 is a two-dimensional photonic crystal having a refractive index periodic structure in the X and Z directions and having a uniform refractive index in the Y direction. In SiO2 having a refractive index of 1.45, spherically-shaped particles of Si3N4 (refractive index of 2.0) having a radius of 0.113 μm are three-dimensionally arranged at an interval of 0.25 μm to form a square lattice, and nine Si3N4 layers are formed in the light traveling direction.
FIG. 12 shows the dispersion plane of the blue photonic crystal 107 having such a structure. That is, the blue photonic crystal 107 has a substantially square dispersion plane for the light having a wavelength of 500 nm, which is substantially blue. When blue light is incident on the photonic crystal 107 having such a dispersion plane, the light travels in the direction perpendicular to the dispersion plane independent of the angle of incidence. In the solid-state imaging element 200, since the optical receiving element 102 is formed in the direction perpendicular to the dispersion plane for blue, blue light incident on the photonic crystal 107 will be parallel guided light in the photonic crystal 107, and the parallel guided light in its entirety is directed to the optical receiving element 102.
In FIG. 10A, incident light 109 is focused by the microlens 106, so that the spot diameter of the focused light decreases as the light passes through the color filter layer 105 and the acryl layer 104. However, once in the photonic crystal 107, the entire light is bent in the direction perpendicular to the optical receiving surface of the optical receiving element 102, so that the entire incident light passes through the opening of the light-blocking layer 103 and reaches the optical receiving element 102.
On the other hand, in FIG. 10B, oblique incident light 110 is focused by the microlens 106 and the spot diameter of the focused light decreases, as described above. When the focused light reaches the interface of the photonic crystal 107, the traveling direction of the light is directed in the direction perpendicular to the optical receiving surface of the optical receiving element 102. Thus, most of the oblique incident light 110 can also be focused onto the optical receiving element 102.